Cifrar un archivo BMP Ejercicio VB.Net - Curso de Programación Visual Basic

Imports System
Imports System.IO
Namespace Bmp
    Class Program
        Private Shared Sub Main(ByVal args As String())
            Dim myFile As FileStream
            Console.Write("Enter the name of file: ")
            Dim fileName As String = Console.ReadLine()

            If Not File.Exists(fileName) Then
                Console.WriteLine("The file not exists!!!")
                Return
            End If

            Try
                myFile = File.Open(fileName, FileMode.Open, FileAccess.ReadWrite)
                Dim b1 As Byte = CByte(myFile.ReadByte())
                Dim b2 As Byte = CByte(myFile.ReadByte())

                If (Convert.ToChar(b1) <> "B"c) OrElse (Convert.ToChar(b2) <> "M"c) Then
                    Console.WriteLine("This File is NOT a BMP file")
                Else
                    myFile.Seek(0, SeekOrigin.Begin)
                    myFile.WriteByte(CByte("M"c))
                    myFile.WriteByte(CByte("B"c))
                End If

                myFile.Close()
            Catch e As Exception
                Console.WriteLine("Error: {0}!!!", e.message())
            End Try
        End Sub
    End Class
End Namespace